Re: Using lasers to communicate

[lwp@conch.aa.msen.com (Lou Poppler)]

This idea of sending data via laser beams across open spaces has some
very useful potential.  I want to suggest some motivation and some
enhancement.  [quotes below give a little background, from the list]

Eavesdropping and channel-blocking and physical-location-discovery are 
related threats to which most traditional data channels are susceptible.  
Any link which depends on a physical conduit (phone line, fiber, coax)
is relatively easy to interrupt and to trace to its end points.
RF links, even with frequency hopping, are subject to triangulation and
jamming.  All these kinds of links can be eavesdropped.

Point-to-point conduitless laser signalling, as envisioned by "Bill" and
Tim in their quotes below, eliminates or reduces these threats. 
Now consider an enhancement.  In show business, we sometimes entertain
the folks with "laser light shows".  The technology used is fairly
straightforward, mainly involving the use of mirrors (the effect also uses
smoke ususally, but please don't prematurely dismiss my remarks on this
basis).  The laser source is attached to a "laser table" which holds a 
number of small mirrors which may be individually inserted (via fast
solenoids) into the path of the laser beam.  Each of these mirrors is then
calibrated to aim at a particular place in the theatre, usually another
larger mirror.  Then (under computer control) the various small mirrors
on the laser table are rapidly inserted and withdrawn from the light beam,
causing the laser beam to follow first one path, then another, then another
through the (smoky) air -- all to the delight of the audience.

This technology could easily be adapted to make a communication channel
safer from the various threats of eavesdropping, interruption, and tracing.
A single point-to-point channel could be made to follow various paths 
having common elements only VERY close to the endpoints.  Better still,
a network of more than two nodes could be constructed without needing to
provide multiple transceivers at each node (and with possibly multiple 
beam paths between each pair).  With known methods of routing and
collision avoidance, we could thus not only route around any known opposition
but also make it very expensive to eavesdrop or even to discover that 
a signal exists.  ("Honey, call the EPA again -- those gubmint boys are back,
driving their oil-burning old van around Mr. May's house.").

[previous attribution unknown...:]
} >>With a tightly focused beam (light is easy, I don't know about lower
} >>frequencies), you can prevent interception except by very obvious physical
} >>devices.  (e.g. Someone in a cherry picker truck.)  You may be able to
} >>avoid the need to encrypt the link (and all the paranoia about key
} >>management, advances in factoring etc. that that implies.)
} >>
} >>Bill

On Thu, 4 Jan 1996 12:45:15 -0800, tcmay@got.net (Timothy C. May) wrote:
} 
} Just a couple of points on this optical idea.
} 
} We were linking buildings a mile apart in the 70s, at Intel. We needed to
} ship CAD data back and forth, and PacBell rates for a dedicated line were
} outrageous, slow to be installed, etc. So, a commercially available laser
} and modulator/demodulator (modem, but it bears sometimes using the longer
} version, to remind people of what it is doing in general) were mounted on
} the roofs of our buildings. I'm sure various packages are commercially
} available to do this.
[snip] 
} I'm actually more positive on low-level (below safety regs get interested
} in) light than on free space RF, for bypassing of the local cable/phone
} monopolies. There's just not enough "bandwidth of free space" available. Do
} the math.
[snip]


::::::::::::::::::::::::::::::::::::::
::  Lou Poppler <lwp@mail.msen.com> ::  " The more you drive, 
::      http://www.msen.com/~lwp/   ::    the less intelligent you are."
::::::::::::::::::::::::::::::::::::::    -- Repo man